Hydrogenation processes leading from biomolecules to fossil biomarkers in anoxic sediments are crucial for the preservation of organic matter. However, these processes are still poorly understood. The present identification of several reduced carotenoids in recent sediments attests that these processes operate at the earliest stages of diagenesis without structural or stereochemical specificity, implying a nonbiological reduction pathway. Sulfur species (e.g., H2S) are the hydrogen donors involved in such reduction, as demonstrated with laboratory experiments. These reactions allow the preservation of abundant organic carbon in the rock record.
Over 2 x 10 9 STB of oil and 8.9 x 10 12 SCF of gas have been produced from the chalk fields of the Greater Ekofisk area, more than 50% of that being produced from the giant Ekofisk Field. Phillips Petroleum Company Norway and the PL018 licence co-venturers have undertaken a comprehensive and integrated re-evaluation of the chalk exploration potential in Blocks 2/4 and 2/7 of the Norwegian Continental Shelf. Recent exploration models indicated the potential for significant untested reserves to exist in the chalk of Blocks 2/4 and 2/7. The Chalk Exploration Project (CEP) team was given the objective 'to define the risked value of the remaining chalk exploration potential in licence PL018'. All major structures and significant amplitude anomalies at chalk level had been drilled by 1986, leaving only the subtle field flank and stratigraphic plays for evaluation. Although many studies had been completed in the chalk over the years since discovery of Ekofisk Field in 1969, most of these studies had been focused on a single field and no regionally consistent stratigraphic framework or depositional model had been developed. As part of this project, a robust regional model for chalk prospectivity was developed covering the whole chalk basin, based on a sequence stratigraphic analysis of regional seismic and well databases, together with petrophysical, hydrodynamic, geochemical and other key technical studies. Acquisition of 3D seismic data over the chalk fields since the mid 1980s demonstrated the power of 3D seismic analysis to define stratigraphic detail within the chalk, and by 1993 the entire licensed area of Blocks 2/4 and 2/7 was covered by 3D data, giving the opportunity to undertake a semi-regional, sequence stratigraphic 3D analysis of the chalk reservoir in PL018, and determine the remaining exploration potential. This paper describes the results of the 'Chalk Exploration Project' (CEP) a study performed by Phillips Petroleum Company Norway together with their Norwegian PL018 coventurers and concludes by denning the seven key risk factors for evaluating subtle chalk stratigraphic prospects and leads.In order to leverage the extensive chalk expertise available within the licence group, a team of geoscientists was assembled from PL018 co-venturer companies in early 1995. The study area was centred on the Ekofisk Field and included the whole of PL018 (Fig. 1). The project had three main phases; database compilation, development of a regional exploration model and lead/prospect analysis and risking. The development of a regionally standardized well and seismic database was a significant challenge, but this provided the foundation for the project and where possible all well data were captured in digital format. The well database was extensive, comprising exploration and development data from over 500 wells. The main components of the database were: (a) a regional, phase-corrected, 2D seismic grid and a semi-regional 3000 km 2 merged 3D seismic survey; (b) a regional velocity database; (c) a regional biostratigraphic...
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